neuro anatomy and physiology of masticatory system / orthodontic courses by indian dental academy

INDIAN DENTAL ACADEMY

Leader in continuing dental education www.indiandentalacademy.com

www.indiandentalacademy.com

NEURO ANATOMY AND NEURO ANATOMY AND PHYSIOLOGY OF PHYSIOLOGY OF

MASTICATORY SYSTEMMASTICATORY SYSTEMThe function of masticatory system is

complex Discriminatory contraction of the various

head and neck muscles is necessary to move the mandible precisely and allow effective functioning.


A highly refined neurologic control system regulates and coordinates the activities of the entire masticatory system .

It consist of primarily nerves and muscles hence the term neuromuscular system.

Muscles:Motor unit: It is the basic component of

neuromuscular system –consist of number of muscles fibers that are innervated by6 one motor neuron .

Each neuron joins with the muscle fiber at the motor end plate


Activation of neuron – Motor end plate is stimulated to release small amounts of acetylcholine-

initiates depolarisation of muscle fibers –

muscle fibers to shorten or contract. Number of muscle fibers innervated by one

motor neuron varies greatly according to the function of motor unit.


Fewer muscle fibers /motor neuron-more precise movement

Eg.ciliary muscles(2or3fibers/neuron-controls of eye-1neuron/1oomuscle fibers-rectus femorus in leg)

Similarly muscles of mastication The inferior lateral pterygoid muscle has relatively

low muscle fiber-motor neuron ratio therefore capable of doing more precise adjustments in length needed to adopt to horizontal changes in mandibular position


In contrast-masseter-large no of muscle fibers/motor neuron-more gross functions of providing the force necessary during mastication

Muscle:Hundred to thousand of motor units along

blood vessels and nerves are bundled together by connective tissue and fascia to make up muscle.


To understand the effect of these muscles one must observe the basic skeletal relationships of head and neck.

The skull is supported in position by cervical spine in fact cervical spine alone cannot support the head so far this muscles are needed to balance it.

If the head is to be maintained in an upright position so one can see forward’,the muscles that attach the posterior aspect of skull to the cervical spine and shoulder region must contract.


Some of the muscles that serve this function are the trapezius,sternocliedomastoid,splenus capitus and long capitus muscles.

It is possible, however for these muscles to over contract and direct the line of vision too far upward.

To counteract this action,an antagonistic group of muscles exists in anterior region of the head –masseter (joins mandible to skull),suprahyoid muscles and infrahyoid muscles.


– Muscle function: motor unit can carry out only one action I.contraction or shortening

– Muscle –3 potential functions– Isotonic contraction-when large no of motor

units in the muscle rare stimulated –contraction or an overall shortening of muscle occurs-this type of shortening under a constant load

– Eg.isotonic contraction occurs in the masseter muscle when the mandible is elevated .forcing the teeth through bolus of food.


Isometric contraction-when proper no of motor units contract opposing a given force,the resultant function of muscle to hold or stabilize the jaw. This contraction without shortening is called isometric contraction eg.it occurs in masseter muscle when an object is held between the teeth (pipe or pencil)

Controlled relaxation:when stimulation of motor unit is discontinued,the fibers of motor unit relax and return to their normal length.


By control of this decrease in motor unit stimulation,a precise muscle lengthening can occur that allows smooth and deliberate movement.

Eg.masseter muscle when the mouth opens to accept a new bolus of food during mastication.

When these three functions are used, the muscles of head and neck maintain a constant desirable head position


Therefore muscles may act as primary movers of part,they may act in a stabilizing manner so that a movement will be coordinated or smooth or they may act as a fixer of a part so that the part can be maintained firmly in a given position.

These three types of muscle activities are present during routine function of head and neck.


Eccentric contraction-which occurs during certain conditions-which is injurious to the muscle tissue.

It refers to the lengthening of muscle at the same time that it is contracting.

Eg.it occurs with the tissue damage associated during an extension-flexion injury(whip-lash injury)

At the precise movement of motor vehicle accident the cervical muscles contract to support the head and resist movement.


However the impact is great,the sudden change in the inertia of head causes it to move while the muscles contract trying to support it.

Results in sudden lengthening of muscles while they are contracting which results in injury


Neurologic structures:the basic structural unit of the nervous system is the neuron.

Neuron-composed of a mass of protoplasm termed cell body and protoplasmic process from the nerve cell body called axons and dendrites.

Axon-it is the central core that forms the essential conducting part of a neuron and it is an extension of cytoplasm from a nerve cell.


Many neurons are grouped together to form a nerve fiber.

These neurons are capable of transmitting electrical and chemical impulses along their axis enabling information to pass both in and out of the c.n.s

Neurons are classified as-(depending on their location and function)

Afferent neurons-conducts impulse towards C.N.S also called AS sensory or receptors –which receive and convey impulses from receptor organs.


Efferent neurons-conducts it peripherally also called as motor neurons-which convey impulses from c.n.s to effector organ to produce muscular or secretary effects.

Internuncial neurons or interneurons-they provide for alternate or distant connections with many of the cells of the nervous system.

Receptors-they are structures that detect changes in environment within the body and transmit information to c.n.s.


Synapse-where the process of two neurons are in close proximity-consists of section of terminal end of the axon of one neuron with the dendrite zone or cell body of another neuron.

It can be monosynaptic or polysynaptic.Sensory receptors-these are neurologic

structures or organs located in all body tissues that provide information regarding the status of these

tissues to c.n.s by way of the afferent neuron.


Two types of receptors: Exteroreceptors-which mediate sensation of

touch.light,pressure,pain,cold,heat etc. Nocireceptors-specific for discomfort and pain. Proprioceptors-provide information regarding the

position and movement of mandible and associated oral structures.

Mechanoreceptors-respond at a higher frequency to innocous stimuli than to thermal or noxious stimulation they are tactile receptors


Interoreceptors- they carry information regarding the status of the internal organs.

Constant input received from all these receptors allows the cortex and the brain stem to coordinate action of individual muscles or muscle groups to create appropriate response in the individual.


Masticatory system-utilizes four major types of sensory receptors to monitor the status of its structures.

Muscle spindles-specialized receptors organs found in the muscle tissues

Golgi tendon organs-located in tendonsPacinian corpuscles-located in the

tendons.joints.periosteum.fascia and subcutaneous tissues.

Nociceptors-found generally through all the tissues of the masticatory system


Muscle spindles-primarily monitor muscle length

Skeletal muscle consists of two types of muscle fibers that is extrafusal and intrafusal fibers.

Extrafusal fibers-they are contractile and makeup the bulk of the muscle

Intrafusal fibers-minutely contractile.A bundle of intrafusal fibers bound by

connective sheath is called muscle spindle.


Primarily monitor tension within the skeletal muscles.

They are interspersed through the muscles and aligned parallel with the extrafusal fibers.

Within each spindle,the nuclei of intrafusal fibers are arranged in two distinct fashions

Chain like(nuclear type)Clumped(nuclear bag type)


There are two types of afferent fibers that supply the intrafuasal fibers which are classified according to their diameters .

The larger fibers conduct impulses at a higher speed and have lower thresholds.

Primary endings(annulospiral endings)- they end central region of the intrafusal fibers which are larger (1a,a-alpha)

Secondary ending (flower spray endings)- those that end in the poles of spindle away from central region,which are smaller.(2,a-beta)


Since the intrafusal fibers of the muscle spindles are aligned parallel to the extrafusal fibers of the muscles,the intrafusal fibers are stretched as the muscle is stretched.

This stretch is monitored at the nuclear chain and nuclear bag region.


Muscle stretchActivation of annulospiraland flower spray

endingsAfferent neurons carry these neural

impulses to the C.N.S.(the afferent neurons originating in the

muscle spindles of muscles of mastication have their cell bodies in the trigeminal mesencephalic nucleus)


The intrafusal fibers receive efferent innervations by way of fusimotor nerve fibers.

The extrafusal fibers receive innervations by way of efferent motor neurons(most of these have their cell bodies in the trigeminal motor nucleus)

Stimulation of these neurons therefore the group of extrafusal fibers (motor unit )to contraction

Muscle contraction


Golgi tendon organs-they are located in the muscle tendon between the muscle fibers and their attachment to the bone.

They primarily monitor tension.They occur in series with the muscle

spindles.Afferent fibers enter near the middle of the

organ and spread out over the the extent of fibers.


Tension on the tendon stimulates the receptors in the golgi tendon organ.

Therefore contraction of muscle also stimulates the organ.

Likewise,an overall stretching of the muscle creates tension in the tendon and stimulates the organ.


Pacinian corpuscles these are large oval organs made up of concentric lamella of concentric connective tissue .

These are widely distributed because of their frequent location in the joint structures.

They are considered to serve principally for the perception of movement and firm pressure.

They are found in tendons,joints,periosteum,tendinous insertions,fascia and subcutaneous tissue.


Nociceptors: these are sensory receptors that are stimulated by injury and transmit injury information to the c.n.s by way of the afferent nerve fibers.

They are located throughout most of the tissues in the masticatory system.

Several types exist.Some respond exclusively to noxious,

mechanical and thermal stimuli,others respond to wide range of stimuli,from tactile sensations to noxious injury etc.


the nociceptors primarily function to monitor the condition,position and movement of the tissues in the masticatory system

When conditions exist that are either potentially harmful or actually cause injury to the tissues, the nociceptors relay this information to c.n.s as sensations of discomfort or pain.


Reflex action-it is the response resulting from a stimulus that passes as an impulse along an afferent neuron to a posterior root or its cranial equivalent;where it is then transmitted to an efferent neuron leading back to the skeletal muscle.

A reflex action may be monosynaptic or polysynaptic

A monosynaptic reflex occurs when the afferent fiber directly stimulates the efferent fiber in the CNS.


A polysynaptic reflex is present when the afferent neuron stimulates one or more interneurons in the CNS which in turn stimulate the efferent nerve fibers.

Two general reflex actions are important in masticatory system.

Myotactic reflexNociceptic reflex


Myotactic reflex-it is monosynaptic jaw reflex When a skeletal muscle is quickly stretched,this

protective reflex is elicited and brings about a contraction of the stretched muscle.

Sudden stretching of muscle Afferent activity from the spindles Afferent impulses pass into the brain stem to the

trigeminal motor nucleus by way of the trigeminal mesencephalic nucleus where the primary afferent cell bodies are located.


Afferent fibers synapse in trigeminal motor nucleus with efferent motor neurons.

Efferent fibers carry information to extrafusal fibers

Muscle contraction

Nociceptive reflex- it is a polysynaptic reflex to noxious stimuli and hence considered to be protective.


In masticatory system, this reflex becomes active when a hard object is suddenly encountered during mastication.

Unexpected biting on hard objectSudden noxious stimulus is generated by

over loading the periodontal structures (the primary afferent nerve fibers carry information to the trigeminal spinal tract nucleus,where they synapse with interneurons)


These interneurons travel to the trigeminal motor nucleus

Afferent information from the sensory receptors reaches the interneurons,two

distinct actions occur.They synapse with efferent neurons in the

trigeminal spinal tract nucleus


Excitatory interneurons leading to efferent neurons in trigeminal motor nucleus of the jaw opening muscles are stimulated

Causes muscle to contract At the same time-the afferent fibers stimulate

inhibitory interneurons,which cause jaw- elevating muscles to relax.

The overall result is that the jaw quickly drops and teeth are pulled away from the object causing noxious stimuli-this process is called antagonistic inhibition


The myotactic reflex protects the masticatory system from sudden stretching of a muscle.

The nociceptive reflex protects the teeth and supportive structures from damage created by sudden and unusually heavy functional forces .

These are very complex and controlled in higher centers.

Reflex actions play major role in functioning. eg.mastication,swallowing,gagging .coughing,speaking.


Influence from higher centersInfluence from higher centers in brainin brain

Main sensory nucleus trigeminal nerve- Sensory nerve impulses arise from oral receptors

travel to the sensory nucleus of trigeminal nerve. From nerve, the impulses are transmitted to the

motor nucleus of trigeminal nerve and to the cerebral cortex.

Those transmitted directly to the motor nucleus are responsible for involuntary movement.


Mesencephalic nucleus of trigeminal nerve-Changes in tension stimulate proprioceptive

receptors in periodontal membrane,tmj,and muscles of mastication.

Sensory impulses travel to the mesencephalic nucleus in the pons.

From here impulses are transmitted to the motor nucleus and are responsible for involuntary jaw movements.


Motor cortex of cerebrum-impulses arising in the cerebral cortex are carried by nerve fibers to the motor nucleus and are responsible for voluntary jaw movements.

Globus palladius of corpus striatum-this center is responsible for postural adjustments and automatic movements

Cerebellum-impulses from cerebellum go to the motor nucleus and result in synergism of various muscles taking part in functional movements.


Major functions of theMajor functions of the masticatory systemmasticatory system

The neuroanatomy and physiology that has been discussed provide a mechanism by which important functional movements of the mandible can be executed

The functional movements are- Mastication Swallowing Speech Respiration Aimless contact movements


Mastication- it is defined as the act of chewing foods.

This act is made up of rhythmic,well controlled separation and closure of the maxillary and mandibular teeth.

Each opening and closing movement of mandible represents a chewing stroke.

The complete chewing stroke has a movement pattern described as tear shaped


It can be divided into an opening phase and closing phase

Closing phase –crushing phase and grinding phase.

Opening phase-the mandible drops downward from the inter cuspal position to a point where incisal edges of the teeth are about 16 to 18 mm apart.

The mandible then moves laterally 5- 6mm from midline as the closing movement begins.


Closing phase- the first phase of closure traps the food between the teeth and is called as crushing phase.

As teeth approaches each other ,the lateral displacement is lessened so that when the teeth or only 3mm apart the jaw occupies a position only 3-4mm lateral to the starting position of the chewing stroke.

At this point teeth are so positioned that the buccal cusps of mandibular teeth are almost directly under buccal cusps maxillary teeth on the side to which the mandible has been shifted.


As mandible continues to close,the bolus of food is trapped between the teeth which begins the grinding phase of closure stroke.

Grinding phase-the mandible is guided by the occlusal surfaces of teeth back to the inter cuspal position,which causes the cuspal inclines of teeth to pass across each other permitting shearing and grinding of bolus of food.


Tooth contact during mastication-When food is initially introduced into the

mouth, few contacts occur.As the bolus is broken down,the frequency

of tooth contact increases.In the final stages of mastication,just prior

to swallowing contacts occur during every stroke.

Two types of contact have been identified.


Gliding contact-which occurs as the cuspal inclines pass by each other during the

Amount of lateral movement is greater when food is introduced and then becomes less as the food is broken down opening and grinding phases of mastication.

Single contact-which occurs during maximum intercuspal position.


Factors affecting mandibular movements during chewing stroke-

Amount of lateral movement is greater when food is introduced and then becomes less as the food is broken down.

Consistency of food-hard foods –greater amount of lateral movement.

Soft foods-lesser lateral movements eg.chewing on a carrot creates a broader stroke than while chewing on a cheese.


Tall cusps and deep fossa-promote predominantly vertical chewing strokes where as flattened cut worn out teeth encourage a broader chewing stroke.,

Malocclusion produces an irregular and less repeatable chewing stroke

Normal persons with good occlusion masticate with chewing stroke that are well rounded that have definite borders and less repeated.


Persons with t.m.j disorders-strokes are shorter and slower and have an irregular pathway but repeatable pathways related to altered functional movement of the condyle around which the pain is centered.


Thank you

For more details please visit www.indiandentalacademy.com


neuro anatomy and physiology of masticatory system / orthodontic courses by indian dental academy

Documents